The Wight's Chant is an incantation sung by the Barrow-wight when Frodo Baggins, Meriadoc Brandybuck, Peregrin Took, and Sam Gamgee are trapped in its barrow. When Frodo first hears it, it's a song of horrible sounds and cold words that chill him to the bones. The reference near the end to the "dark lord" refers to Sauron and his assumed dominion-to-come, who, sensibly, therefore must have had power over the wights.